1. Field of the Invention
The present invention relates to a color television signal processing apparatus, more particularly to a chromakey apparatus used for insertion of a picture image into another picture image.
2. Prior Arts
Chromakey techniques have been developed and are currently employed, for example, for taking out desired regions with a specified color in a color television image and inserting another image therein. The conventional color image composition processing, i.e. chromakey techniques, employ a keying signal obtained mainly from a difference of hue component contained in a color television signal, among other two color signal components, a luminance and a saturation components. The chromakey techniques are grouped into hard chromakey and soft chromakey techniques depending on whether keying signal characteristics and operations of mixing circuits are a switching type or a linear type, respectively. The soft chromakey technique employes a keying signal (soft key signal) having gradation which linearly gates the mixing circuit in such a manner that the closer the color tones of regions on a color television screen to a base color for extraction are the more the extraction degrees increase. Although this soft chromakey technique can provide for an easier image dissolve than the hard chromakey technique, apparatus carrying out soft chromakey processing inevitably require three R, G, B signals, corresponding to three primary color signals, as their input image signals. This means that the signal input sources are limited to only color cameras. But, in general, color image signals are composite signals made by, e.g. NTSC system, and are transmitted through a single signal line. Therefore, soft chromakey processing using such composite color television signals requires use of R-G-B demodulators in the front stage of the conventional soft chromakey signal generators. A chromakey technique which carries out dissolve processing by use of composite color television signals without decomposition of three color component signals is designated as a line chromakey (encoded chromakey) technique. The line chromakey technique has an advantageous feature that the chromakey processing is possible, when necessary, even after picture recording by use of VTR tapes. Accordingly, the soft chromakey applicable for the line chromakey technique is demanded in various fields of television industry.
The descriptions so far made are of several chromakey processing techniques for the analog composite color television signals. Digital processing for the color television signals have been widely employed so that digital VTR apparatus will become into practical use in the quite near future. In view of these circumstances it is highly desirable to provide chromakey apparatuses which are capable of carrying out signal processing by use of encoded composite color television signals.
On the other hand, the conventional chromakey apparatuses have a number of shortcomings. Manipulation of the selection of the color for regions to be extracted, mainly the selection of the hue level by use of e.g. a hue dial, is very critical, and is liable to be affected frequently and in a complex manner by illumination conditions of a chromakey panel and by time-varying electrical characteristics of the color cameras per se. Also it takes a great deal of time for preparation prior to the acquisition of a desired color image on a monitor television for almost every instance of chromakey processing.